1. Field of the Invention
The present invention relates to an echo sounder transducer which emits an acoustic wave underwater by electroacoustic transduction, and in particular, to an echo sounder transducer suitable for the wideband operation of a Langevin-type echo sounder transducer which enables low frequency operation with securing a sound pressure level beyond a certain level without changing the external dimensions of the echo sounder transducer.
2. Description of the Prior Art
Generally, a Langevin-type echo sounder transducer is known as an echo sounder transducer which emits an acoustic wave underwater by electroacoustic transduction.
The Langevin-type echo sounder transducer is equipped with a bolted Langevin-type vibrator as vibration generating means, and emits an acoustic wave in a predetermined frequency band by applying a compressive stress to a piezoelectric ceramic stacked body sandwiched between a front mass and a rear mass by bolting.
Then, in such the Langevin-type echo sounder transducer, the wideband operation of the echo sounder transducer is achieved without changing the size of a Langevin-type vibrator by further providing bend vibration structure in an acoustic radiation surface side of the front mass of the Langevin-type vibrator, and superimposing a bend vibration mode on a longitudinal vibration mode of the Langevin-type vibrator (refer to patent document 1: Japanese Patent Laid-Open No. 2001-148896 (page 3 to 4, FIG. 1), and patent document 2: Japanese Patent No. 3005611 (page 3 to 4, FIGS. 1 and 2)).
Hereafter, such a conventional Langevin-type echo sounder transducer will be described with reference to drawings. FIG. 11 is a schematic perspective view showing the appearance of the Langevin-type echo sounder transducer, and FIG. 12 is a schematic diagram of the Langevin-type echo sounder transducer in which the bend vibration structure conventionally proposed is provided.
As shown in FIG. 11, a Langevin-type echo sounder transducer 100 is equipped with a Langevin-type vibrator 3 constituted of an active vibrator 9 which is constituted of a piezoelectric ceramic stacked body formed cylindrically and is sandwiched between the front mass 6 and rear mass 7. As shown in FIG. 12, by fastening bolt 8 which combine the front mass 6 and rear mass 7, a compressive stress is applied to the active vibrator 9, and hence, the vibration at a predetermined frequency occurs.
A bend vibrator 2 is installed on a front section of the front mass 6 of the Langevin-type vibrator 3, that is, in an acoustic radiation surface side thereof. The bend vibrator 2 is equipped with a diaphragm 5 which has a cavity therein and can perform bend vibration, and a disciform active vibrator 4 fixed to a back side (front mass 6) of the a diaphragm 5, and by applying a drive voltage, the active vibrator 4 vibrates and the diaphragm 5 performs bend vibration.
In addition, a phase shifter 10 is connected to the cylindrical active vibrator 9 of the Langevin-type vibrator 3, and the disciform active vibrator 4 of the bend vibrator 2 through leads 30, and phase control is performed so that the bend vibration mode of the bend vibrator 2 may become an opposite phase to the longitudinal vibration mode of the Langevin-type vibrator 3.
According to such the conventional Langevin-type echo sounder transducer 100, it is possible to use not only the longitudinal vibration mode of the Langevin-type vibrator 3 but also the bend vibration mode of the bend vibrator 2 by providing the bend vibrator 2 in front of the front mass 6 of the Langevin-type vibrator 3. Hence, it is possible to operate the echo sounder transducer at a low frequency, that is, a bend vibration resonance frequency lower than a longitudinal vibration resonance frequency of the Langevin-type vibrator 3.
In addition, it becomes possible by superimposing the bend vibration mode of the bend vibrator 2 on the longitudinal vibration mode of the Langevin-type vibrator 3 to widen the bandwidth of the echo sounder transducer to achieve wideband operation.
Furthermore, in order to achieve further wideband operation in such a Langevin-type echo sounder transducer, an echo sounder transducer which is formed by dividing a bend vibrator, which is provided on a front section of a front mass of a Langevin-type vibrator, into a plurality of sectors is also proposed (refer to patent document 3: Japanese Patent Laid-Open No. 2003-032771 (page 3 to 4, FIGS. 1 and 2)).
According to such an echo sounder transducer, since a plurality of bend vibrators which are divided into sectors respectively are set to have resonance frequencies different from each other and the bend vibration mode generated by the plurality of these bend vibrators is superimposed, further wideband operation of the echo sounder transducer is achieved.
Nevertheless, in a conventional Langevin-type echo sounder transducer like the above, there arose a problem that, first, in the case of the conventional echo sounder transducer shown in FIG. 12, since the coupling of vibration modes arose because of integral structure of the Langevin-type vibrator and bend vibrator, it was not possible to achieve wideband operation with securing a sound pressure level beyond a certain level.
In the Langevin-type echo sounder transducer being equipped with the bend vibrator, in a vibration mode between two resonance frequencies (in an intermediate area) in the bend vibration mode and longitudinal vibration mode, the bend vibration mode of the bend vibrator and the longitudinal vibration mode of the Langevin-type vibrator occur concurrently, and the two vibration modes are superimposed.
FIGS. 13A to 13C are explanatory diagrams showing schematically the states in respective vibration modes of the conventional echo sounder transducer shown in FIG. 12, and FIG. 13A shows the bend vibration mode, FIG. 13B shows a superimposed vibration mode, and FIG. 13C shows the longitudinal vibration mode. As shown in these figures, in the conventional echo sounder transducer 100 shown in FIG. 12, since the front mass 6 of the Langevin-type vibrator 3 and the diaphragm 5 of the bend vibrator 2 are coupled integrally in a joint 60, the whole bend vibrator 2 vibrates in a longitudinal direction in the form of being combined with the vibration of the Langevin-type vibrator 3 in the longitudinal vibration mode.
For this reason, although the bend vibrator 2 performs the bend vibration in the superimposed vibration mode, as shown in FIG. 13B, a fulcrum in the bend vibration mode moves from reference position ∇ to ▾ in a minus direction. Hence, a portion of vibrating in a positive direction and a portion of vibrating in a negative direction existed in a vibrating surface of the bend vibrator 2 with a neutral surface of acoustic radiation as a border (refer to hatched portions in this figure), as a result, positive and negative acoustic radiation pressures cancelled each other for a transmission level to drop remarkably (refer to “f4” in FIG. 3 described later).
In this way, in the conventional Langevin-type echo sounder transducer, the coupling of vibration modes arose because of integral structure of the Langevin-type vibrator and bend vibrator. Hence, the fulcrum in the bend vibration mode shifted reversely to an amplitude direction of the bend vibrator in the bend vibration mode by the longitudinal vibration mode of the Langevin-type vibrator, that is, in the minus direction largely rather than a reference position. Therefore, there arose a problem that acoustic radiation could not be performed.
For this reason, a sound pressure level in a frequency band (intermediate area) between a resonance frequency of the bend vibrator in the bend vibration mode and a resonance frequency of the echo sounder transducer in the longitudinal vibration mode dropped remarkably, and hence, it was not possible to widen the frequency band so that the sound pressure level beyond a certain level could be obtained.
On the other hand, in the case of the echo sounder transducer which was formed by dividing a bend vibrator into a plurality of sectors, and which is disclosed in patent document 3, the bend vibration mode of a sector-like bend vibrator is generated in a frequency higher than a resonance frequency of a Langevin-type vibrator in the longitudinal vibration mode. Hence, it was possible to widen a frequency band toward a high frequency.
However, in this echo sounder transducer, since the order of a resonance mode being generated became the order from the longitudinal mode to the bend mode, it was necessary to upsize the echo sounder transducer so as to widen the frequency band toward a low frequency.
The present invention is proposed so as to solve the problems which the above conventional technology has, and aims at providing an echo sounder transducer which can widen a using frequency band of the echo sounder transducer from a low frequency to a high frequency while securing a sound pressure level beyond a certain level without changing the external dimensions of the echo sounder transducer by separating the vibration modes of the Langevin-type vibrator and bend vibrator by forming a slit in a front mass of the Langevin-type vibrator.